This invention relates to improvements in surge arrestor devices for protecting an electrical circuit from a lightning strike or other abnormal surge in potential and, in particular, to an arrestor unit which combines both a path to ground and means to equalize the potential between two conductors of the circuit that may be subjected to an excessive voltage differential as a result of the surge.
Air gap arrestors have been widely used for decades in applications where there is a need to protect a circuit involving two conductors, neither of which is electrically connected to earth ground. Since a lightning discharge generally seeks a current path to ground, protecting the circuit requires that a path to ground be provided without grounding the circuit for normal use. Also, due to the presence of two ungrounded conductors, the potential therebetween must be equalized, or the differential minimized, in the event that the two conductors are subjected to unequal levels of coupling from the lightning surge. Otherwise, this can result in an excessive voltage differential between the conductors that may be damaging to the equipment with which the circuit is associated.
Two different types of air gap devices, known as arrestors and equalizers, have been used to provide a ground path for the surge and the necessary equalization of the potential between the two conductors. A typical application is in a railroad signal environment where relatively low-voltage trackside equipment is to be protected. Track circuits are ungrounded, i.e., both conductors are isolated from earth ground. Each conductor is at the potential of a corresponding rail of the track, and both rails may be isolated in a given track section by insulated joints. In a typical installation an air gap arrestor is connected between each conductor and ground. An equalizer is connected across the conductors. Such an arrangement is, for example, shown and described in Rees, U.S. Pat. No. 3,204,322. The resulting configuration, therefore, involves three separate protection devices, two arrestors and an equalizer.
Surge protection has also been provided by gas tube arrestors, a device which in a widely used configuration contains three electrodes enclosed within a sealed chamber. Two electrodes are connected to the respective circuit conductors, and the third electrode is connected to ground. Once an arc is established as a result of a surge, the gas within the chamber becomes highly ionized and the arc expands to contact all three electrodes almost immediately. This brings all three electrodes to the same potential and eliminates the need for the separate equalizer used in the air gap approach.
Although the gas tube arrestor provides all functions within a single unit and eliminates the need for separate components, it does have several disadvantages. A heavy discharge tends to release metallic particles from the electrodes. Since the particles cannot escape from the sealed chamber, a deposit may be formed on the walls of the container and limit the life of the arrestor as eventually a conducting path can be formed between the electrodes. Also, melting of the electrodes can occur and cause a possible short circuit within the tube.